The technical field of the present invention relates to radiation curable compositions. Particularly, it relates to radiation curable coating compositions containing unsaturated polyester compounds in combination with vinyl ether compounds.
There are a number of coating applications requiring the use of coating compositions that cure rapidly, adhere well to a substrate and do not discolor during the curing process. In some instances, radiation curable coating compositions can be used to meet these objectives.
The basic chemistry of radiation curing involves a rapid polymerization of unsaturated materials initiated by free radicals formed when the system is irradiated by ionizing radiation and/or actinic radiation (hereinafter individually and collectively referred to as "radiation"). One of the major components of many conventional radiation curable coating compositions are acrylates or methacrylates (hereinafter individually and collectively referred to as "(meth)acrylates").
Notwithstanding the widespread use of radiation curable coating compositions which include (meth)acrylates as one of their major components, there are problems associated therewith. For example, their cure response is significantly inhibited by the presence of oxygen in the curing atmosphere. Moreover, there are also concerns with the possibility of skin and eye irritation sometimes associated with their the used and/or handling. For these and other reasons, it is becoming desirable in the coating industry to reduce and/or eliminate the amount of (meth)acrylates present in radiation curable coating compositions.
One alternative to such conventional radiation curable coating compositions is disclosed in commonly-owned U.S. Pat. No. 5,536,760. Specifically, that patent discloses radiation curable coating compositions having, as their major components, ethylenically unsaturated ester compounds having a plurality of unsaturated ester groups and polymerizable vinyl ether compounds. In one preferred embodiment of that patent, the unsaturated ester compounds are polyunsaturated polyesters having more than 2.0 unsaturated ester groups per molecule.
Coatings produced in accordance with U.S. Pat. No. 5,536,760 have a relatively fast cure response when exposed to radiation. If a large number of ethylenically unsaturated groups are present in the unsaturated ester compounds, the coatings disclosed therein have the potential of being hard and brittle. Although there are many applications where properties such as hardness and brittleness are highly desirable, it is sometimes equally desirable for radiation-cured coating composition to be hard and flexible. Moreover, notwithstanding their relatively fast cure response, it is also often desirable to even further accelerate the cure rates of such coating compositions.
Often times, accelerating cure responses reduces the coating composition's stability. For example, if accelerated cure responses are achieved by selecting and/or employing components that are more readily polymerizable when exposed to radiation, these same components are typically more susceptible to self polymerization and/or to initiate the polymerization of other components prior to radiation exposure. On the other hand, if components are selected and/or employed that are less readily polymerizable until exposed to radiation, these same components typically reduce the coating composition's cure response.
U.S. Pat. No. 3,814,702 pertains to unsaturated polyester-based radiation curable coatings containing a photosensitizing composition which includes: a highly sensitive polymerization initiator (i.e., a benzoin compound), an organic acid which activates the benzoin compound, and a solvent capable of solubilizing the benzoin compound and the acid. According to that patent, the addition of such a photosensitizing composition to the unsaturated polyester-based coating improves the coating's cure response and stability. That patent also discloses that a weak reducing agent (e.g., an organic phosphite compound) added to the photosensitizing composition can improve the coating's stability, as well as enhance the performance of the photosensitizing composition.
Contrary to the disclosure in U.S. Pat. No. 3,814,702, U.S. Pat. No. 4,116,788 discloses that, in customary unsaturated polyester-based coatings which contain a benzoin photosensitizing composition, photopolymerization time is not shortened by adding an organic phosphite compound. The radiation curable coating compositions disclosed in U.S. Pat. No. 4,116,788 contain (meth)acrylic acid esters as one of their major components. According to that patent, the addition of a benzoin compound and an organic phosphite compound to the radiation curable compositions disclosed therein improves the coating's cure response and stability.